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The *Shanghan Lun* (Treatise on Cold Damage) contains discussions on the deficiency and excess of Yin and Yang, where the subtle meanings are not without analytical clarification. The *Nan Jing* (Classic of Difficult Issues), Chapter 48, states: "In terms of deficiency and excess in diseases, outward movement indicates deficiency, while inward movement indicates excess." Generally, when the True Yang of the exterior is deficient, Yin pathogens become excessive, moving outward to attack Yang. Consequently, the pulse floats on the exterior, the disease location is in the exterior, and the treatment principle should be diaphoresis (sweating therapy). When Yin pathogens move to the exterior and the pulse floats, one must not be confused into thinking this indicates an Excess of Yang pulse.

When the True Yin within is deficient, Yang pathogens become excessive, moving inward to attack Yin. Consequently, the pulse sinks inward, the disease location is in the interior, and the treatment principle should be purgation. When Yang pathogens move into the interior and the pulse is solid inside, one must not be confused into thinking this indicates an Excess of Yin pulse. This statement is not merely arbitrary theory established by ancients; rather, it aims to teach that if there is Excess of Yin, one should suppress Yin and assist Yang; if there is Excess of Yang, one should suppress Yang and assist Yin.

If Yin is excessive, causing pathogens to manifest outwardly, external-releasing medicines should have a warm nature to assist Yang Qi, such as Guizhi Tang (Cinnamon Twig Decoction). If Yang is excessive, causing pathogens to accumulate inwardly, interior-purging medicines should have a cold nature to suppress Yang Qi, such as Chengqi Tang (Gut-Purging Decoction).

One might ask: "Yin pathogens moving outward to attack the exterior indicates a deficiency of Yang. Since this is a Yang disease, should it not be treated with diaphoresis? Why emphasize this?" This is entirely incorrect. If Yin pathogens transmit to the exterior, how can they leave without diaphoresis? The nature of Guizhi Tang is warm; using warmth is precisely to assist Yang. When Yang is assisted and grows, the Yin pathogens are thereby eliminated. Herbs with pungent and sweet flavors that disperse belong to Yang; this is what is meant here. Zhang Zhongjing's statement that "Chengqi Tang entering the stomach causes Yin excess to perish" is precisely to warn against mistakenly using Chengqi Tang for purgation when Yin excess manifests outwardly, which would inevitably lead to demise.

Alternatively, one might ask: "Yang pathogens moving inward to attack the interior indicates a deficiency of Yin. Since this is a Yin disease, should it not be treated with warming nourishment and purgation? Why emphasize this?" This is also incorrect. If Yang pathogens enter the interior, how can they exit without purgation? The nature of Chengqi Tang is cold; using coldness is precisely to suppress Yang. When Yang is suppressed, it becomes mild, allowing True Yin to grow. Herbs with sour and bitter flavors that induce vomiting or purging belong to Yin; this is what is meant here. Zhang Zhongjing's statement that "Guizhi Tang swallowed causes death when Yang is excessive" is precisely to warn against mistakenly using Guizhi Tang for diaphoresis when Yang excess moves inwardly, which would inevitably lead to death.

Observing that ancient external-releasing medicines were mostly warm and interior-purging medicines were mostly cold, one can understand that the subtle meaning of Yin-Yang deficiency and excess lies not merely in the application of sweating methods, but is specifically established to guide the choice between warm and cold medicinal properties.